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(#) `remember` calls must not return `Unit`

!!! ERROR: `remember` calls must not return `Unit`
   This is an error.

Id
:   `RememberReturnType`
Summary
:   `remember` calls must not return `Unit`
Severity
:   Error
Category
:   Correctness
Platform
:   Any
Vendor
:   Jetpack Compose
Identifier
:   androidx.compose.runtime
Feedback
:   https://issuetracker.google.com/issues/new?component=612128
Min
:   Lint 8.0 and 8.1
Compiled
:   Lint 8.0 and 8.1
Artifact
:   [androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.md.html)
Since
:   1.5.0
Affects
:   Kotlin and Java files and test sources
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/main/java/androidx/compose/runtime/lint/RememberDetector.kt)
Tests
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/RememberDetectorTest.kt)
Copyright Year
:   2020

A call to `remember` that returns `Unit` is always an error. This
typically happens when using `remember` to mutate variables on an
object. `remember` is executed during the composition, which means that
if the composition fails or is happening on a separate thread, the
mutated variables may not reflect the true state of the composition.
Instead, use `SideEffect` to make deferred changes once the composition
succeeds, or mutate `MutableState` backed variables directly, as these
will handle composition failure for you.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/androidx/compose/runtime/foo/FooState.kt:14:Error: remember calls
must not return Unit [RememberReturnType]
    remember {
    --------
src/androidx/compose/runtime/foo/FooState.kt:17:Error: remember calls
must not return Unit [RememberReturnType]
    val unit = remember {
               --------
src/androidx/compose/runtime/foo/FooState.kt:21:Error: remember calls
must not return Unit [RememberReturnType]
    remember(unitLambda)
    --------
src/androidx/compose/runtime/foo/FooState.kt:22:Error: remember calls
must not return Unit [RememberReturnType]
    val unit2 = remember(unitLambda)
                --------
src/androidx/compose/runtime/foo/FooState.kt:28:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number) {
    --------
src/androidx/compose/runtime/foo/FooState.kt:31:Error: remember calls
must not return Unit [RememberReturnType]
    val unit = remember(number) {
               --------
src/androidx/compose/runtime/foo/FooState.kt:35:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number, unitLambda)
    --------
src/androidx/compose/runtime/foo/FooState.kt:36:Error: remember calls
must not return Unit [RememberReturnType]
    val unit2 = remember(number, unitLambda)
                --------
src/androidx/compose/runtime/foo/FooState.kt:42:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number1, number2) {
    --------
src/androidx/compose/runtime/foo/FooState.kt:46:Error: remember calls
must not return Unit [RememberReturnType]
    val unit = remember(number1, number2) {
               --------
src/androidx/compose/runtime/foo/FooState.kt:51:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number1, number2, unitLambda)
    --------
src/androidx/compose/runtime/foo/FooState.kt:52:Error: remember calls
must not return Unit [RememberReturnType]
    val unit2 = remember(number1, number2, unitLambda)
                --------
src/androidx/compose/runtime/foo/FooState.kt:58:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number1, number2, number3) {
    --------
src/androidx/compose/runtime/foo/FooState.kt:63:Error: remember calls
must not return Unit [RememberReturnType]
    val unit = remember(number1, number2, number3) {
               --------
src/androidx/compose/runtime/foo/FooState.kt:69:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number1, number2, number3, unitLambda)
    --------
src/androidx/compose/runtime/foo/FooState.kt:70:Error: remember calls
must not return Unit [RememberReturnType]
    val unit2 = remember(number1, number2, number3, unitLambda)
                --------
src/androidx/compose/runtime/foo/FooState.kt:76:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number1, number2, number3, flag) {
    --------
src/androidx/compose/runtime/foo/FooState.kt:81:Error: remember calls
must not return Unit [RememberReturnType]
    val unit = remember(number1, number2, number3, flag) {
               --------
src/androidx/compose/runtime/foo/FooState.kt:87:Error: remember calls
must not return Unit [RememberReturnType]
    remember(number1, number2, number3, flag, calculation = unitLambda)
    --------
src/androidx/compose/runtime/foo/FooState.kt:88:Error: remember calls
must not return Unit [RememberReturnType]
    val unit2 = remember(number1, number2, number3, flag, calculation = unitLambda)
                --------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/androidx/compose/runtime/foo/FooState.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package androidx.compose.runtime.foo

import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember

class FooState {
    fun update(new: Int) {}
}

@Composable
fun Test() {
    val state = remember { FooState() }
    remember {
        state.update(5)
    }
    val unit = remember {
        state.update(5)
    }
    val unitLambda: () -> Unit = {}
    remember(unitLambda)
    val unit2 = remember(unitLambda)
}

@Composable
fun Test(number: Int) {
    val state = remember { FooState() }
    remember(number) {
        state.update(number)
    }
    val unit = remember(number) {
        state.update(number)
    }
    val unitLambda: () -> Unit = {}
    remember(number, unitLambda)
    val unit2 = remember(number, unitLambda)
}

@Composable
fun Test(number1: Int, number2: Int) {
    val state = remember { FooState() }
    remember(number1, number2) {
        state.update(number1)
        state.update(number2)
    }
    val unit = remember(number1, number2) {
        state.update(number1)
        state.update(number2)
    }
    val unitLambda: () -> Unit = {}
    remember(number1, number2, unitLambda)
    val unit2 = remember(number1, number2, unitLambda)
}

@Composable
fun Test(number1: Int, number2: Int, number3: Int) {
    val state = remember { FooState() }
    remember(number1, number2, number3) {
        state.update(number1)
        state.update(number2)
        state.update(number3)
    }
    val unit = remember(number1, number2, number3) {
        state.update(number1)
        state.update(number2)
        state.update(number3)
    }
    val unitLambda: () -> Unit = {}
    remember(number1, number2, number3, unitLambda)
    val unit2 = remember(number1, number2, number3, unitLambda)
}

@Composable
fun Test(number1: Int, number2: Int, number3: Int, flag: Boolean) {
    val state = remember { FooState() }
    remember(number1, number2, number3, flag) {
        state.update(number1)
        state.update(number2)
        state.update(number3)
    }
    val unit = remember(number1, number2, number3, flag) {
        state.update(number1)
        state.update(number2)
        state.update(number3)
    }
    val unitLambda: () -> Unit = {}
    remember(number1, number2, number3, flag, calculation = unitLambda)
    val unit2 = remember(number1, number2, number3, flag, calculation = unitLambda)
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/RememberDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `RememberDetector.returnsUnit`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=612128.

(##) Including

!!!
   This is not a built-in check. To include it, add the below dependency
   to your project.

```
// build.gradle.kts
implementation("androidx.compose.runtime:runtime-android:1.8.0-alpha07")

// build.gradle
implementation 'androidx.compose.runtime:runtime-android:1.8.0-alpha07'

// build.gradle.kts with version catalogs:
implementation(libs.runtime.android)

# libs.versions.toml
[versions]
runtime-android = "1.8.0-alpha07"
[libraries]
# For clarity and text wrapping purposes the following declaration is
# shown split up across lines, but in TOML it needs to be on a single
# line (see https://github.com/toml-lang/toml/issues/516) so adjust
# when pasting into libs.versions.toml:
runtime-android = {
    module = "androidx.compose.runtime:runtime-android",
    version.ref = "runtime-android"
}
```

1.8.0-alpha07 is the version this documentation was generated from;
there may be newer versions available.

NOTE: These lint checks are **also** made available separate from the main library.
You can also use `androidx.compose.runtime:runtime-lint:1.8.0-alpha07`.


[Additional details about androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.md.html).
(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("RememberReturnType")
  fun method() {
     remember(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("RememberReturnType")
  void method() {
     remember(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection RememberReturnType
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="RememberReturnType" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'RememberReturnType'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore RememberReturnType ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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